Thesis: S.B., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2018. / Cataloged from PDF version of thesis. / Includes bibliographical references (page 35). / Grinding and Polishing of small parts is often easily performed by human hands, yet is challenging to automate. The grinding and polishing process is best done using a force-control scheme, which human hands perform naturally. Heavy robotic arms, which favor a position-control scheme, are difficult to control precisely, and trajectory errors can cause excessive grinding force which leads to burning of the part or destruction of the grinding wheel. Prior art of direct force control on a large robot arm requires the end-effector to have a 6-axis dynamometer, which is unwieldy, costly, and greatly limits the speed/precision of the process. We will discuss a new type of grinding spindle which is axially compliant, allowing the position-control robot arm to be used in a force-control nature. The spindle has a disjoint force-displacement curve, effectively operating in two modes: position-control mode at first, until a critical force is exceeded, when the spindle transitions into force-mode, keeping constant grinding force on the part though a certain range of travel. This limits the amount of force which can be imparted during grinding to a safe amount. The spindle is very simple and mechanically robust. We have built this hybrid position-force control spindle and tested it. The spindle was shown to perform correctly and successfully completed the test grind. / by Austin Brown. / S.B.

Massachusetts Institute of Technology. Dept. of Mechanical Engineering. Thesis. 1966. Sc.D. / Bibliography: leaves 48-51. / Sc.D.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 1987. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND ENGINEERING / Bibliography: leaves 132-133. / by Robert Eric Robinson. / M.S.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2012. / Cataloged from PDF version of thesis. / Includes bibliographical references (p. 39). / The purpose of this research is to develop a design tool for geared actuation systems that experience bidirectional exchange of energy with the environment. Despite the asymmetry of efficiency depending on the direction of power transfer in geared systems, typical dynamic models consider a fixed transmission efficiency for all conditions which can result in significant error depending on specific gear selection and the number of stages. This error can cause issues especially in dynamic legged robots and haptic devices when accurate force control is desired. In this paper we present directional impedance, a characteristic of geared transmissions in which the amount of power loss through the transmission differs according to the direction of power flow. Typical robots use electric motors with high gear reduction which introduces larger impedance when power flows from the output back to the motor than when the power flows from motor to output. To investigate the dependence on power flow direction, friction loss from gear teeth sliding in the gear mesh is modeled by a single gear tooth contact model and dynamic models are presented for each power transfer direction. Combinations of 0.5 mod gears were tested in experiment over a range of sizes between 16 and 120 teeth to characterize the directional effects over multiple gear selections. The experiments confirmed that for a set of differently sized gears, power loss is greater when the larger gear drives the smaller one than in the reverse case, and the asymmetry was up to 17% in the 16 and 120 tooth gear set. With a multiple stage gearbox, the difference in loss is further amplified. These findings show that directional loss in gears is a non-negligible effect and must be considered in both dynamic modeling and gear selection of robotic actuators. The gear loss model enables the modeling of motor and gearbox as a single package which can then be optimized for desired performance parameters such as peak torque, torque per mass, and mechanical impedance. / by Albert Duan Wang. / S.M.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2002. / Includes bibliographical references (p. 147-154). / The cooling of a thin, moving glass fiber was studied through both experiments and theoretical method in the present thesis. An experimental system was built at a laboratory scale, which included a glass fiber production subsystem, a temperature measurement subsystem, and a thermocouple temperature control subsystem. A heated thermocouple technique was adopted to measure the temperature distribution of the glass fiber along its drawing direction. Data were collected for diameters ranging from 20 to 50 micrometers and speeds from 1 meter per second to 6 meters per second. Experiments were performed both with and without water spray cooling of the fiber. A comprehensive analysis was performed to estimate the uncertainty in our experiments. The analysis shows that, without water spray, the 2a uncertainty is 14.8%, and with water spray, it is 15.3%. The major uncertainty comes from the uncertainty of the thermocouple probe. For theoretical modeling, the von Karman-Pohlhausen boundary layer integral technique was used to predicting the cooling rate of the fiber. The model considers the effects of water spray, variation of drawing speed, fiber diameter, environment parameters, and initial conditions, extending earlier work on the subject. The comparison between the experimental data and the theoretical prediction shows integral methods produce the correct trends, but show systematic disagreements with the data. The models with and without spray show similar levels of disagreement. The direction and magnitude of these disagreements are system dependent. / (cont.) Potential causes may include fiber vibration effects, boundary layer transition, and measurement uncertainties. Thus, future work should focus on measuring/modeling the vibration effect and determining the amplitudes/frequencies of the vibration (which are expected to be system dependent). Incorporation of spray dispersion effects is also required for improved modeling. / y Daxi Xiong. / Ph.D.

Thesis: S.B., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2014. / Cataloged from PDF version of thesis. / Includes bibliographical references (page 21). / Different fabrication methods are evaluated for producing pressure-compensating tubes for use in low-pressure drip irrigation systems. Such devices would allow drip irrigation systems to operate at driving pressures much lower than those required by current available technology, allowing for cost and water savings in developing nations. Fabrication methods explored consist mainly of molding of liquid silicone rubber and production from existing silicone stock by folding or binding multiple sheets of material together. Based on small-scale production and testing, the sheet fabrication method showed some promise of equaling the performance of the injection-molded tubes, but both methods require further refinement going forward. / by Ian Martin. / S.B.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 1989. / Includes bibliographical references (leaves 209-212). / by Tetsuro Butsuen. / Ph.D.

Thesis (S.M. and S.B.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2005. / Includes bibliographical references (leaf 44). / Bearing setting has a significant impact on bearing life and system performance. A strain based load measurement technique that has been previously developed and patented will be used to measure preload in automotive drive axle pinion bearings. This technique utilizes strain gages placed in notches that have been machined into the outer diameter of a bearing cup. The performance of two gage types, semiconductor and metal foil, are compared. Preload in the pinion bearings is simulated in a machine by applying an axial load to a bearing assembly. The bearing assembly is rotated at various speeds and temperatures for each set of axial loads. Two notch geometries are compared for each set of tests conducted. An effort has been made to develop a finite element model that can be used to verify the results of the gage calibration. The results of the calibration reveal that metal foil and semiconductor gages can provide preload measurements that are accurate to within 3.7% and 7.7% of the full scale input load (73 lbs. and 155 lbs.) respectively. / by Anthony H. Thomas, II. / S.M.and S.B.

Thesis: S.M., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2013. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 67-71). / Crystalline silicon solar cells are a proven renewable energy technology, but they have yet to reach low costs commensurate with subsidy-free, grid-scale adoption. To achieve the widespread adoption of photovoltaics, the cost per unit of electricity must be reduced by increasing solar cell efficiency. Parts per trillion concentrations of iron impurities in the silicon material can severely limit solar cell efficiency. Iron can be found in both precipitated and point defect form in silicon. Both forms are detrimental to final solar cell efficiency, but their negative impact can be mitigated during solar cell processing. In a standard solar cell process, the phosphorus diffusion step is the key opportunity to redistribute iron impurities because it is the step with the largest thermal budget. Phosphorus diffusion process optimization for solar cell material so far typically consists of one or more isothermal steps followed by a cooling step. Iron silicide precipitates can be dissolved at high temperatures, whereas at lower temperatures, interstitially dissolved iron is driven to the phosphorus-rich layer. Previous optimizations typically maximize minority carrier lifetime without constraining process time and device parameters. This thesis explores a novel phosphorus diffusion process in which there are no isothermal steps. The goal of this work is to demonstrate simultaneous maximization of minority-carrier lifetime, while maintaining high process throughput and steady emitter sheet resistance. Predictive simulation, electrical characterization techniques, and synchrotron-based X-ray fluorescence were combined to compare this new processing approach to standard solar cell processing. This continuously ramped temperature processing may be a promising approach for maximizing solar cell performance, maintaining reasonable manufacturing rates, and achieving a target sheet resistance. / by Ashley E. Morishige. / S.M.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 1996. / Includes bibliographical references (p. 76). / by Mark Edward Ellis. / M.S.